1. Technical Field of the Invention
The present invention relates generally to the field of multiple output dcxe2x80x94dc converters. More particularly, the present invention relates to the regulation of multiple outputs in flyback converters.
2. Description of the Related Art
Conventional switching power supplies commonly include a power transformer and one or more power switches for alternately coupling an unregulated DC or rectified AC voltage across a primary winding of the power transformer in a series of voltage pulses. These pulses are converted into a series of voltage pulses across one or more secondary windings of the power transformer and then rectified and filtered to provide one or more output DC voltages. The output voltage or voltages of the power converter are commonly regulated by controlling the relative amount of time that the power switch is on (i.e., the duty cycle).
One common type of switching power supply is the flyback power converter, which is an isolated version of the buck-boost converter. A flyback power converter works by cyclically storing energy in a coupled inductor, and then dumping this stored energy into a load. By varying the amount of energy stored and dumped per cycle, the output power can be controlled and regulated. A switching transistor connected in series with the primary winding of the power transformer normally provides such a switching function. That is, the on-time and off-time of this power switch controls the amount of energy stored in the coupled magnetic field. When the power switch is on, current flows through the primary winding of the power transformer, and energy is stored in the transformer. When the power switch is off, the stored energy is transferred out into a secondary circuit by means of current flowing out of one or more secondary windings of the power transformer. Note that the secondary current does not flow in the power transformer at the same time that the power switch is on and the primary current is flowing. The reason for this is that in a conventional flyback power converter, the primary-secondary winding polarity is chosen such that a rectifier when coupled to the secondary winding will prevent conduction of current in the secondary winding when the power switch is on.
Flyback power converters are advantageous at lower power levels over other switching power converters due to the fact that they are generally simpler, they require a reduced number of components, and they allow multiple regulated outputs to be available from a single supply. Common applications for flyback converters are AC adapters, which may, for example, deliver an output voltage in the range of between 9 VDC to 180 VDC at power levels of 20 to 100 Watts, drawing power from a rectified AC mains, which may vary between 85 VAC to 410 VAC for universal mains input.
Flyback converters are generally operated in one of two modes, a first mode, referred to as discontinuous conduction mode, well known in the art, in which the energy stored in the transformer is totally coupled to the output load before the next energy cycle, generally resulting in the secondary current reaching zero before the next drive cycle. The second mode of operation is referred to as continuous conduction mode, whereby the next energy cycle begins before all stored magnetic energy is released from the transformer, and therefore before the secondary current reaches zero. Discontinuous conduction mode is more common than continuous conduction mode because relatively simple control circuitry can be used to maintain output voltage regulation by varying the frequency and/or on-time of the power switch to accommodate heavy or light load conditions.
The control circuitry for either DCM or CCM operation is somewhat similar. In the case of CCM, the loop bandwidth must generally be much lower to avoid instability due to the right hand plane zero (RHPZ) from the output diode. Because of its higher bandwidth capabilities, DCM is often preferred. For high current applications, CCM is generally preferable because the root-mean-square (RMS) currents (hence losses) are lower.
FIG. 1 illustrates a conventional active clamp flyback converter having multiple secondary outputs and an active snubber network on the primary side to protect the main switch SM. In a conventional flyback converter, all of the outputs are xe2x80x98onxe2x80x99 simultaneously. The ratio of the output voltages on the different windings is determined by the winding ratio and the actual relative voltage levels are determined by the xe2x80x98onxe2x80x99 time of the main switch.
FIG. 2 is an illustration of another conventional flyback converter having sequenced multiple secondary outputs sourced from a single secondary winding. The circuit of FIG. 2 further includes a passive resistor-capacitor-diode (RCD) snubber network formed by capacitor Cs, resistor Rs, and Diode DS on the primary side to protect the main switch SM. Such networks are well known in the art.
The circuit of FIG. 2 operates in accordance with a simple sequential pulse-width modulation (PWM) scheme for controlling the xe2x80x98onxe2x80x99 time of the respective outputs. PWM schemes are well known in the art. A drawback associated with the circuit configuration of FIG. 2 concerns certain limitations imposed on the output voltage control.
In particular, if the converter operates in a continuous mode (DCM), then all of the energy stored in the transformer T during the primary switch Sm xe2x80x98onxe2x80x99 time must be transferred to the secondary outputs 211, 213, 215 during each switching cycle.
In the circuit, each of the outputs on the secondary side is locally controlled with the exception of the last or final output. The final output (e.g., output 215) is indirectly controlled by the xe2x80x98onxe2x80x99 time of the primary side switch SM. That is the xe2x80x98onxe2x80x99 time of the primary switch SM must be of such duration to guarantee that there is sufficient energy stored in the transformer during the charging cycle to maintain a voltage V3 across R3 at a pre-determined level, irrespective of the xe2x80x98onxe2x80x99 time of switch SM. It is noted that irrespective of how long a time switch SM is maintained in the xe2x80x98onxe2x80x99 state, if there is insufficient energy stored in the transformer in the first instance, once the discharge cycle is initiated maintaining switch SM xe2x80x98onxe2x80x99 for a longer duration cannot correct for an initially insufficient amount of energy stored in the transformer to maintain the third output at the proper voltage level. For precise secondary side control, this is not a desirable limitation. Additionally, this restriction imposes a further drawback requiring that there be a xe2x80x9csmartxe2x80x9d controller on the primary side.
It is therefore an object of the present invention to provide an improved power converter having each of a plurality of outputs regulated exclusively from the secondary side quasi-independent of the xe2x80x98onxe2x80x99 time of the primary switch.
In accordance with the present invention, a multiple output flyback converter is provided. The converter provides independently controlled high gain-bandwidth secondary side voltage regulation to compensate for rapid small signal changes at the multiple outputs on the secondary side. In a preferred embodiment, the converter further includes a low gain-bandwidth slow loop feedback controller connecting the secondary and primary sides for responding more slowly to large signal changes.
The converter according to the present invention includes a transformer, or coupled inductors, having a primary winding and a secondary winding. The secondary winding includes first and second terminals. An input circuit is connected to the primary winding for coupling an input DC voltage to the primary winding. The input circuit includes an active clamp circuit for recovering both energy in the leakage inductance, as is performed in prior art circuits, and in addition, recover residual energy in the magnetic field of the secondary winding of the transformer at the end of each energy cycle.
In accordance with one embodiment, multiple identically constructed output circuits are connected in parallel to the first and second terminals of the secondary winding. Each output circuit includes a switch connected in series with a diode, an energy storage capacitor connects the diode/switch pair to the return lead. Each output circuit further includes a fast feedback loop (i.e., secondary side controllers) which maintains a regulated output voltage. The fast feedback loop compensates for small changes in the load on the order of 1 to 5 percent.
The multiple output flyback converter of the present invention, in addition to providing secondary side controllers for maintaining a regulated output voltage, also requires a primary controller for controlling the primary switch xe2x80x98onxe2x80x99 time to supply the output requirements.
An advantage of the circuit of the present invention is that with secondary-side control, the accuracy and bandwidth of the primary loop can be relaxed. The primary controller is responsible for maintaining the required amount of energy xe2x80x98overdrivexe2x80x99 to allow small-signal control of the last secondary winding output. This can be accomplished in a number of different ways. One way is to monitor the current level in the last secondary winding at the time of turn-off via an opto-coupler. Alternatively, the primary controller could monitor secondary turn-off by the voltage change on the drain of the primary switch.
Another advantage of the circuit of the present invention is that small load changes are rapidly and precisely compensated for exclusively on the secondary side via fast local feedback loops for each output. This feature overcomes the limitation of the prior art in which the last or final output is controlled from the primary side.
A further advantage of the circuit of the present invention is that the local feedback loops on the secondary side provide more precise voltage control than can be achieved with prior art configurations.
A still further advantage of the circuit of the present invention is that excess or residual energy stored in the secondary winding of the coupled winding is returned to the source via the primary circuit via the active clamp network. A still further advantage of the circuit of the present invention is that soft-switching of primary side switches is achieved via the active clamp.